Role of Grid Decarbonization
It’s no secret that emissions from a building’s energy use are inherently tied to the “cleanliness” of the grid. With increasing urgency surrounding anthropogenic climate change, there has been action to pursue less environmentally-taxing energy alternatives. In 2018, only 18% of U.S. electricity generation came from renewable resources however, while the use of electricity will continue to rise, the switch to renewables is projected to amount to 31% of U.S. electricity generation by 2050. The switch to renewables naturally have the greatest impact on the more energy-intensive processes of operating a building, such as regulating ambient temperature, managing plug loads, and providing adequate lighting. But these changes, specifically electricity’s fuel source, also have a direct impact further upstream at the inception of a material’s life cycle. Steel and concrete, for example, are two energy-intensive materials that are commonly specified in construction. An important question to think about is how the gradual decarbonization of the grid will ultimately impact the carbon footprint of these materials and subsequently how they are used in construction projects.
Up to 96% of CO2 emissions of a concrete mix are attributable to the cement content. The culprit is the calcination of calcium carbonate accounting for close to 60% of concrete’s total CO2 emissions. In this process, pulverized rock is heated in a kiln resulting in the desired clinker, a chemical binding of the input material. The thermal decomposition naturally produces CO2 as a result, thus acting as a significant limiting factor in the ability to reduce concrete’s carbon footprint. Other contributors to CO2 output in the manufacturing of concrete accounting for the remaining 40% include aggregate production, concrete plant operations, kiln fuel, and transportation. These CO2 emissions are byproducts of combustion, a chemical reaction which occurs when using the cement kiln and transporting resources to the cement plant. Most of the energy used to make concrete (as much as 88%) is from non-renewable fuels which is harnessed primarily for the manufacturing of cement. While the energy used for both concrete plant operations and concrete manufacturing is included in the Life Cycle Inventory (LCI) by Marceau et. al (2007), the resources needed to create this electricity and fuels (known as the upstream profile) is intentionally excluded. Concrete will always be limited by the inherent chemical reaction necessary to form cement and the environmental burden from any electricity used in the formation of concrete is small enough to be considered negligible.
The two main methods in which steel is produced are the blast furnace/basic oxygen furnace route (BOF) and electric arc furnace (EAF) route. The EAF process uses scrap steel melted via high-current electric arcs while BOF steelmaking blasts oxygen to remove impurities from molten iron to convert it into steel. One of the primary outputs from both EAF and BOF steelmaking is hot rolled coil which is typically further processed for use in other applications.
“In the U.S., all hot-rolled sections are produced using scrap based electric arc furnaces.” Approximately 98% of the primary energy demand (PED) of the formation of hot rolled coil is from non-renewables. Upstream processes, including electricity generation, are close to 100% responsible for the PED and are responsible for approximately 35% of the global warming potential (GWP) of hot rolled coil. Similar findings from the American Institute of Steel Construction (AISC) Environmental Product Declaration (EPD) for fabricated hot-rolled structural sections reveal that the raw materials supply stage, which includes upstream activities, has the highest PED and accounts for 85%-95% of impact assessment categorizes such as climate change potential and ozone depletion.
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According to the World Steel Association LCI Study (2018), it is evident that “steel production is an energy-intensive industry and therefore the consumption of energy and electricity are one of the main contributors to the environmental impact of the steelmaking process.” Given that electricity is a critical input, steel’s environmental impact is a direct function of the grid’s energy source. It will also be location dependent as different countries and regions have a distinct electricity grid mix.
Grid decarbonization will likely have positive impacts on the steel carbon footprint, given steel’s high reliance on electricity to transform the raw material into its structural form. The same cannot be said for concrete whose production is fuel-intensive, but mostly independent of the grid and includes emissions due to the calcination process.
It is evident that a material’s embodied carbon is also a function of upstream energy generation—a compelling reason for requesting EPDs from material manufacturers. The distinction between industry-average data and producer-specific data should be made when soliciting environmental impact documentation. For steel, the current de factor standard is to provide industry-average values, given the variation of processes across fabricators. Nonetheless, asking for product- and supplier-specific data is an important tool the specifier can use to influence individual manufacturer choices and to spur manufacturing innovation.
It is also important to remember that steel and concrete are only two of hundreds of materials that we use in construction and there are other considerations aside from electricity usage that should be weighed in order to understand their contributions on a larger scale. Transportation arrangements, project schedule and cost, and social equity implications should all be part of the holistic evaluation of a given material. If we want to decarbonize our buildings, our first step should be to consider the materials we specify and the environmental effects of their inception.